Yolk utilization in stick insects entails the release of vitellin polypeptides into the perivitelline fluid

This study investigates the developmental fate of vitellin (Vt) polypeptide s generated by limited proteolysis in an insect embryo. To this end, a numb er of polyclonal (pAb) and monoclonal antibodies (mAb) were raised against the yolk sac and the perivitelline fluid of late embryos of the stick insec t Carausius morosus. Two dimensional immuno gel electrophoresis and Western blotting demonstrate that polypeptides resulting from Vt processing are pr esent both in the yolk sac and the perivitelline fluid. At the confocal mic roscope, different labelling patterns were detected in the ooplasm dependin g on the stage of development attained by the embryo. At early developmenta l stages, label is associated with large unsegmented portions of the fluid ooplasm. During embryonic development, the fluid ooplasm is gradually trans formed into yolk granules by intervention of vitellophages. Prior to dorsal closure, the yolk sac is separated from the perivitelline fluid by interpo sition of serosa cells (the so called serosa membrane). Several mAbs raised against the perivitelline fluid react specifically with this membrane sugg esting that the release of Vt polypeptides from the yolk sac occurs by intr acellular transit through the serosa cells. By immunocytochemistry, gold la bel appears associated with the cell surface and a number of vacuoles of th e serosa membrane. These data are interpreted as suggesting that Vt polypep tides resulting from limited proteolysis in stick insect embryos are not ex haustively degraded within the yolk sac, but are instead transferred transc ytotically to the perivitelline fluid through the serosa membrane.